Honestly its not a lot of work to take a spoke off and measure it properly. Get several spare spokes, and note down the length for next time.
If you measure a non-drive side spoke off the rear wheel (they're easier to get off, remember the driveside ones are slightly shorter, by about 1~2mm but many rear wheels can get away with the same length.
There are a bunch of spoke-length calculators online (here's one). This tells you what your spoke length should be, not what it is. You'll need to have exact measurements for the current hub and rim, although the calculator I linked to has a database of measurements you can look up.